In light-emitting diode applications, dimmable front-end devices having a large input and output voltage range are increasingly demanded. Thus, in light-emitting diode applications, an adjustable power range is demanded which distinctly exceeds an adjustable power range of a conventional front-end device for fluorescent lamps. Although there are also front-end devices which can dim various loads to a respective part-power of about 1%, these front-end devices must always additionally provide a basic power for heating the electrical coils of the fluorescent lamps so that converter stages of the front-end devices must always produce a corresponding minimum power which is rarely less than 10% of the design power.
In the case of front-end devices for solid-state lighting (SSL), this is different. In this case, a power range is frequently demanded which is adjustable to up to 100 mW or up to below 1% of the design power. In this context, US 2012/0286686 A1 discloses a lighting facility for a solid-state light source.
SSL front-end devices frequently have energy converters or converters, respectively, which are operated in switching mode, i.e. are clocked. In order to be able to generate the very low powers, the converters are frequently operated at very high frequencies. This causes not only high switching losses but can additionally also cause problems in regard to the electromagnetic compatibility (EMC).
Furthermore, the operation at high frequencies requires power components suitable for this and possibly corresponding circuit measures which are associated with high costs. For this reason, such converters for providing very low powers are operated, for example, in step-up mode or also in step-down mode below a predetermined reference power in a so-called burst mode. The burst mode is characterized by the fact that the converters remain connected for some clock cycles and are thereafter deactivated for a longer period, that is to say a number of clock cycles. The burst mode is found to be disadvantageous in as much as a noise development due to frequencies within the audible range which result from the sequence of burst packets can be the consequence and an intermediate circuit of such a converter can have increased ripple due to the operation following the principle of a 2-point regulator.
In a known embodiment of such a lighting facility, an additional supplemental switch-off time which, for example, is called a pause time, is inserted after a switch-off time given by the complete discharge of an energy storage device. This pause time is inserted after each clock cycle so that the duration of a period of the drive clock is extended.
In particular, the switch-on period of the electronic switching element can be set to a fixed value in a particular power range. During an operation in accordance with such a method, however, compromises can be found with respect to power loss, a constant steady transmission characteristic curve, the accuracy of the output voltage and of the power range in which the lighting facility can be operated in accordance with predetermined boundary conditions. Since light-emitting diodes are light sources having very short response times, special demands are made here on the constancy of the transmitted power. Furthermore, it is of great importance especially for dimming the lighting device that the transmission characteristic curve of the energy converter extends steadily and particularly monotonously so that the transmitted power of the energy converter also follows a change of the predetermined dimming level directly.